1. Field of Invention
This invention relates to fence mounted intrusion detection devices, and has particular reference to an intrusion detection alarming device which extends the height of a security fence or security barrier and utilizes the supporting member of the extension as an alarming arm so that an alarm is triggered when weight or force is applied to any element of the extended barrier.
2. Description of Prior Art
Present intrusion detection systems have their sensing elements including trip wires mounted along security fence fabric or other security barriers and utilize the structural members of the barrier as a means of support. These same structural members can also be used as a means of support by intruders as they scale or climb over the security barrier just so long as they do not disturb the detector elements supported by the structural members.
Most of the penetration of areas protected by fences or barriers are a result of intruders scaling or climbing over the fences or barriers rather than by cutting or breaking through them. However, most of the present intrusion detection systems are mounted on the fences or barriers in a manner designed to primarily detect penetration through the center of the barrier rather than detecting a person or persons scaling or climbing over the barrier.
Present intrusion detection systems which utilize electronic detection schemes are susceptible to false alarms and require constant maintenance to make sure the fence is tight plus constant adjustment of the electronic processors in order to keep the system operational.
One class of electronic intrusion detectors register the motion of the fence fabric if a person attempts to cut through or climb over the fence. The detector elements are designed to mount on the fence fabric which exhibits more motion when the fence is disturbed than do the supporting members which are normally fence posts. U.S. Pat. No. 4,365,239 issued Dec. 21, 1982 to Ronald W. Mongeon, is representative of this type of detector which employs microphonic coaxial cable stretched longitudinally along the fence as the sensing element. Electrical noise is generated as the fence fabric is moved and this noise is processed in an attempt to distinguish the sounds generated when someone climbs the fence versus the noise generated when the fence fabric is moved by weather conditions such as wind or rain. The result is a high number of false alarms. When this system is in service, it requires constant maintenance on the fence fabric and the electronics to keep the system operational. This detector system is installed on the fence fabric which is supported by the fence posts and totally ignores the extended barrier which, when utilized, is mounted on the top of the posts by means of rigid arms. The extended barrier arm can be used by an intruder as a non alarming supporting member while he climbs over or scales the fence just so long as he does not kick the fence fabric or move it in a violent manner.
There are also other types of fence mounted intrusion detectors such as U.S. Pat. No. 3,237,105 issued Feb. 22, 1966 to Henry P. Kalmus, and U.S. Pat. No. 4,064,499. issued Dec. 20, 1977 to Theodore D. Geiszler and Ronald W. Mongeon. These systems employ long wires suspended from the fence posts which transmit and receive radio frequency energy. This energy is absorbed when a person enters the field and this change in energy level is monitored by the receiving wire and processed to annunciate an alarm. These systems also have high false alarm rates since the slightest movement of the long wires will also vary the amount of energy present at the receiver. The systems require constant maintenance to adjust the tension of the detector wires. This system totally ignores the extended barrier which is mounted on the fence posts by means of rigid arms. The rigid extended barrier arms can be used by an intruder as a non alarming supporting member as he climbs over or scales the fence. The grounded fence fabric usually shields the intruder from the detection field which is usually mounted inside the fence surrounding the protected area.
There is a mechanical system U.S. Pat. No. 4,533,906 issued Aug. 6. 1985 to Yoel Amir and U.S. Pat. No. 4,683,356 issued Jul. 28, 1987 to Aric Stoler which employs a number of long lengths of barbed wire strung longitudinally along the length of a fence and are used as trip wires. The trip wires are firmly anchored at the ends of the detection zone and the detection alarming devices are mounted in the center of the zone. An alarm is generated when one or more of the barbed wires is moved by a person attempting to climb or crawl through the barbed wire barrier. The trip wires are maintained under tension and require massive end posts making the system very expensive to install and maintain. The system is susceptible to false alarms since the trip wires are fabricated from ferric metals and have a high coefficient of expansion which results in changes in the tension of the wires under normal temperature changes from daylight to darkness. This change in tension can move the detection alarming device which will register it as an alarm. The trip wires which tie to the detection alarming devices must be supported by rigid posts. Extended barriers are normally mounted on rigid arms which can be used by an intruder as a non alarming supporting member while he climbs or scales the fence just so long as he does not put any weight or pressure on the trip wires.
All of the systems described above have a very difficult time in protecting gates with the result that gates are usually protected by detectors such as microwave or infrared systems which are not installed on the fence or barrier but are usually installed behind the fence line.
Razor ribbon concertina is sometimes employed along the top of security fences since it presents a very visible deterrent. Concertina is difficult to alarm and none of the systems described above will work as an alarm system for concertina.